Multi-realm Keystone

Currently, in order to make use of a given OpenStack service—when Keystone is in use—, a token issued by Keystone must be presented in the request. This falls short of the federation goal, where a customer running an OpenStack instance could issue a request to a contracted service provider also running OpenStack, using their existing authentication tokens. In this proposal, I put forward a mechanism by which this federation can be accomplished, along with a terminology that can help simplify future discussions.